Applications of nanostructured semiconductors can be economically pursued with the use of semiconductor-grade wafers as a source of silicon if the product can be produced in no other way, e.g., integrated circuits, or if the product cost is not a factor, e.g., highly specialized applications such as aerospace, deep space, military, and some medical applications. However, numerous applications of nanostructured semiconductors, e.g., consumer products, high-energy materials, lighting, secondary (e.g., lithium ion) batteries, sensors, and thermoelectric materials, would benefit from economical production of porous powder on the kilogram to ton scale. Porous silicon in particular has been demonstrated to have especially appealing properties to be used in theranostics (Wang, C.-F., Sarparanta, M. P., Mäkilä, E. M., Hyvonen, M. L. K., Laakkonen, P. M., Salonen, J. J., Hirvonen, J. T., Airaksinen, A. J. & Santos, H. A., Multifunctional porous silicon nanoparticles for cancer theranostics, Biomaterials, 48, 108-18 (2015)), nanomedicine (Fontana, F., Mori, M., Riva, F., Mäkilä, E., Liu, D., Salonen, J., Nicoletti, G., Hirvonen, J., Caramella, C. & Santos, H. A, Platelet Lysate-Modified Porous Silicon Microparticles for Enhanced Cell Proliferation in Wound Healing Applications, ACS Appl Mater Interfaces, 8, 988-96 (2016)), drug delivery (Zhang, H., Liu, D., Shahbazi, M.-A., Mäkilä, E., Herranz-Blanco, B., Salonen, J., Hirvonen, J. & Santos, H. A., Fabrication of a multifunctional nano-in-micro drug delivery platform by microfluidic templated encapsulation of porous silicon in polymer matrix, Adv. Mater., 26, 4497-4503 (2014)), and biomedical imaging (Santos, H. A., Bimbo, L. M., Lehto, V.-P., Airaksinen, A. J., Salonen, J. & Hirvonen, J., Multifunctional Porous Silicon for Therapeutic Drug Delivery and Imaging, Current Drug Discovery Technologies, 8, 228-49 (2011)), if only the porous silicon can be made in bulk quantities.
Electroless etching of metallurgical-grade Si ($1 kg−1 versus $10,000 kg−1 for semiconductor-grade Si) is recognized as a process with tremendous industrial potential but only if issues related to reproducibility, controllability, purity, cost, and scaling can be addressed. Li, X., Lee, J.-H., Sprafke, A. N. & Wehrspohn, R. B., Black metallurgical silicon for solar energy conversion, Semicond. Sci. Technol., 31, 014009 (2016); Chadwick, E. G., Mogili, N. V. V., O'Dwyer, C., Moore, J. D., Fletcher, J. S., Laffir, F., Armstrong, G. & Tanner, D. A., Compositional characterisation of metallurgical grade silicon and porous silicon nanosponge particles, Rsc Adv, 3, 19393-402 (2013); and Loni, A., Barwick, D., Batchelor, L., Tunbridge, J., Han, Y., Li, Z. Y. & Canham, L. T., Extremely High Surface Area Metallurgical-Grade Porous Silicon Powder Prepared by Metal-Assisted Etching, Electrochem. Solid State Lett., 14, K25-K27 (2011). HNO3-based processes suffer from an inability to produce specific surface areas greater than 150 m2 g−1, incomplete etching of particles, and a yield of 5%. Chadwick et al. (2013); Chadwick, E. G., Beloshapkin, S. & Tanner, D. A., Microstructural characterisation of metallurgical grade porous silicon nanosponge particles, J. Mater. Sci., 47, 2396-2404 (2012); and Limaye, S., Subramanian, S., Goller, B., Diener, J. & Kovalev, D., Scaleable synthesis route for silicon nanocrystal assemblies, Phys. Status Solidi A, 204, 1297-1301 (2007).
A major advance in stain etching was the discovery by Kurt W. Kolasinski and co-workers (Nahidi, M. & Kolasinski, K. W., The effects of stain etchant composition on the photoluminescence and morphology of porous silicon, J. Electrochem. Soc., 153, C19-C26 (2006); and Dudley, M. E. & Kolasinski, K. W., Stain etching with Fe(III), V(V) and Ce(IV) to form microporous silicon, Electrochem. Solid State Lett., 12, D22-D26 (2009)) that HNO3 could be replaced by oxidants that produce significantly less gas during etching. Replacement of HNO3 by Fe3+ led to production of powders with much greater specific surface area (up to 408 m2 g−1) and improved yield (η≤0.24, defined as the ratio of final mass to initial mass). Loni et al. (2011); Wang, M., Hartman, P. S., Loni, A., Canham, L. T. & Coffer, J. L., Stain Etched Nanostructured Porous Silicon: The Role of Morphology on Antibacterial Drug Loading and Release, Silicon, 8, 525-31 (2016). Such replacement failed to resolve, however, issues with process control (particularly regarding thermal budget, drying, and salt precipitation, see U.S. Pat. No. 9,540,246) and cost. Kolasinski and co-workers further demonstrated that V2O5, which is not a metal salt but an oxide, dissolved in HF etches Si without a concentration threshold, without an induction time, that the oxidant it produces in solution is optimally coupled to the Si valence band for maximum etch rate, and that with control of gas production rate and drying conditions homogeneous films of roughly 20 μm depth could be obtained on etched wafers. Kolasinski, K. W., Gogola, J. W. & Barclay, W. B., A test of Marcus theory predictions for electroless etching of silicon, J. Phys. Chem. C, 116, 21472-81 (2012); Kolasinski, K. W., Charge Transfer and Nanostructure Formation During Electroless Etching of Silicon, J. Phys. Chem. C, 114, 22098-05 (2010); and Dudley et al. (2009).
In U.S. Patent Application Publication No. 2004/0166319, Li et al. describe a porous silicon powder comprising individual silicon particles wherein only the outermost layer of each individual particle is porous. The porous layer has a maximum thickness of only 500 nm. Farrell et al. report, in International Patent Application Publication No. WO 2007/037787, etching porous silicon particles that comprise a solid core surrounded by a porous silicon layer having a thickness greater than about 0.5 microns. In the disclosed processes a stain etching method is used. In neither of these patent references were the porous silicon particles etched completely to the core.
In U.S. Patent Application Publication No. 2009/0186267, Tiegs describes using the method of Farrell et al. to produce an anode in a lithium ion battery. Canham and Aston disclose, in U.S. Patent Application Publication No. 2008/0260839, performing stain etching after lithographic patterning of a silicon wafer to produce porous silicon cubic particles. Sheem et al. (U.S. Patent Application Publication No. 2004/0214085) and Canham and Friend (U.S. Patent Application Publication No. 2016/0308205) produced non-luminescent porous silicon particles by acid leaching of the metal component, e.g., Al, of a metal/silicon alloy particle. A family of patents obtained by Green et al. (e.g., U.S. Pat. No. 9,184,438 and related EP Patent No. 2 321 441) disclose having produced pillared silicon particles by metal-assisted catalytic etching (MACE) of silicon powders in which the pillars are crystalline.